the FAO is the organ oft he UN family which is recommending limits:
http://www.fao.org/home/en/
or search for certain metals or food additives on their codex alimentarius: http://www.fao.org/fao-who-codexalimentarius/
Then, there are the EU regulations that you might find helpful: Commission Regulation (EC) N.° 629/2008 of 2 July 2008 amending Regulation (EC) N.° 1881/2006 setting maximum levels for certain contaminants in foodstuffs. Official Journal of the European Communities, L 173 (2008), pp. 6-9 or any newer version thereof. The EU Regulation sets maximum levels (MLs) for total mercurycontent (THg) for fish and seafood from 0.5–1.0 mg/kg wet weight , while the FAO/WHO has established provisional tolerable weekly intake (PTWI) for MeHg as the amount of contaminant (e.g., MeHg that can be ingested over a lifetime without appreciable risk. The PTWI value is calculated for a week due to the bioaccumulation of the toxic elements in humans. This value for MeHg is 1.6 μg/kg bw/week, for THg is 4 μg/kg bw/week.
There has also been a very similar question in this forum in 2017, where you could look at,